1. Field of the Invention
This invention relates to immobilized catalysts on inorganic carriers such as silica, their preparation and their use for disproportionation of chlorosilicon hydrides to silane.
2. Description of the Prior Art
The addition of chlorosilicon hydrides to various unsaturated organic compounds offers a synthetic route to many organochlorosilanes which are useful for many industrial purposes (E. Y. Lukevitts and M.. G. Voronkov, "Organic Insertion Reactions of Group IV Elements", Consultants Bureau, New York, 1966). In particular, trichlorosilane is commercially used as the starting material for the production of very high purity silicon for the electronics industries (F. A. Padorvani, U.S. Pat. No. 4,092,466). The pyrolysis of chlorosilane to metallic silicon is known as Siemens Process. Extensive developmental work has been recently conducted to reduce the energy costs for that process by substituting trichlorosilane with other silanes such as dichlorosilane or silane which decompose at lower temperature than trichlorosilane (L. H. Coleman, U.S. Pat. No. 4,340,574). Union Carbide Corporation has patented a process which incorporates redistribution of chlorosilanes to silane and silane decomposition.
Chlorosilicon hydrides such as chlorosilanes can be prepared by reacting metallic silicon directly with hydrogen chloride in the presence of copper catalyst. This process is commercially performed using a fluidized-bed reactor to control the reaction temperature, because it is exothermic. The reaction temperature is controlled carefully to maximize the yield of trichlorosilane, for otherwise, tetrachlorosilane will be the major product. With careful control of the process, trichlorosilane can be obtained up to 80% of the products and silicon tetrachloride will be about 15%. However, the reaction gives only trace amount of dichlorosilane which is expected to be the major product. This is why dichlorosilane is usually prepared by redistributing trichlorosilane (C. J. Litteral, U.S. Pat. No. 4,113,845).
The Lewis acid type catalysts such as aluminum trichloride, boron trichloroide, etc. are reported to be active for the disproportionation of chlorosilanes, e.g. see U.S. Pat. Nos. 2,627,451 and 2,735,861. Organic compounds such as tertiary amines, quaternary ammonium compounds, nitrile compounds, phosphines, etc. are also suggested in U.S. Pat. Nos. 2,732,282 and 3,928,542 as catalysts for the reactions, such as the following: ##STR1##
The reaction, however, requires the reaction temperatures as high as 200.degree. C. and the reactor must be kept under high pressure because of the low boiling points of the chlorosilanes. Temperatures greater than 300.degree. C. are also required for the catalysts of alkaline metal salts, as shown by M. Kinger in U.S. Pat. No. 3,627,501. Because of the harsh conditions of temperature and pressure, these inorganic compounds are unsuited for a continuous type industrial process.
Although the reaction proceeds at relatively lower temperature with organic catalysts, than with inorganic catalysts, very efficient distillation is required to separate the organic catalyst from the products since the organic catalysts are usually used as homogeneous, soluble catalysts.
Union Carbide's U.S. Pat. No. 3,928,542 to C. Bakey and U.S. Pat. No. 4,113,845 to C. Litteral disclose the use of various, solid amine ion exchange resins for the disproportionation of chlorosilane, thereby immobilizing the catalyst for hetergeneous catalysis. One of the important, immobilized catalysts developed commercially for this purpose is Amberyst A-21, a trade mark of Rohm and Hass company, Philadephia Pa. This catalyst is a macroreticular styrene divinylbenzene copolymer resin bearing pendant benzyldimethylamine groups. Union Carbide Corporation has also patented the processes for the redistribution of trichlorosilanes, using Amberyst A-26 or Amberitre IRA-400, which are ion exchange resins that are exchanged with quarternary ammonium compounds.
The commercial catalyst for the disproportionation of chlorosilanes has several disadvantages. Because the catalyst is a benzyl amine or ammonium complex, gradual degradation takes place due to the loss of the amine group from the benzyl sites. In addition, the organic backbone of a styrene divinylbenzene copolymer is susceptible to swelling and shrinking. This mandates very careful control of the composition of reaction stoichiometry and temperature to prevent restrictions in flow through the reactor catalyst beds.
Inorganic materials such as silica, zeolite, etc. have hydroxyl groups on the surface that can be used as the site to couple with organotrialkoxysilanes, as suggested by F. R. Hartley and P. N. Vezey, Adv. in Organometal. Chem., V 15, 189 (1978). Treating inorganic fillers with coupling agents is being commercially practiced in the plastic industries.